1. Field of the Invention
The present invention relates to circuit protection in television and video display monitors, more particularly to protection of EHT (electrical high tension) and/or scan output stages in multiscan displays.
2. State of the Art
In television and video display monitors using a CRT (cathode ray tube), a horizontal deflection current is produced in a horizontal deflection coil to create a magnetic field used to deflect an electron beam (produced by a cathode of the CRT) back and forth across a display screen. As the electron beam is scanned back and forth across the display screen, it is modulated to produce on the display screen spots of varying luminous intensity, thus forming an image to be viewed. A high voltage, referred to as EHT (electrical high tension), is applied to an anode of the CRT and accelerates the electron beam, causing it to strike the screen at a very high speed.
Typically, a periodic horizontal drive signal is applied to a horizontal scan output stage (i.e., a power transistor) to produce a train of high-voltage retrace pulses used to generate the horizontal retrace current. The same train of retrace voltage pulses may also be used to generate the EHT using a voltage step-up transformer, commonly referred to as the flyback transformer. Where high performance is not required, as in television sets and some less-expensive video monitors, a single output stage is used to generate the horizontal retrace current and to generate the EHT, with the flyback transformer and the retrace coil being combined on a common core as part of a single assembly. In high performance video monitors, separate scan and EHT output stages are provided, with the retrace coil and the flyback transformer also being separated. Power is provided to the scan and EHT output stages from a so-called B+ power supply.
In a multiscan monitor, the horizontal drive frequency can vary between, say, 30 kHz and 80 kHz. At lower horizontal frequencies, current builds up in the primary of the flyback transformer for a longer period of time while the EHT output stage is turned on. When the EHT output stage is turned off, therefore, a larger voltage pulse is generated than in the case of a higher horizontal frequency. If the power supply to the EHT output stage were to remain the same throughout the range of possible horizontal frequencies, the EHT would then be frequency dependent, resulting in performance variations.
Furthermore, even in the case of a single horizontal frequency, variations in picture content and consequent variations in beam current may also produce variations in the EHT. For example, the top half of a picture frame might be black (no beam current) and the bottom half of the picture frame might be white (high beam current). During high beam current, the EHT is loaded down and reduced, for example by 1-2 kV. As a result, the picture becomes wider in the white are than in the black area, creating picture distortion. Therefore, it is common for the B+ power supply to the scan and EHT output stages to be varied using a step-down power supply and a control loop in order to maintain a fairly constant EHT.
Failure of certain components may cause an excessive amount of current to be drawn through the step-down power supply, possibly damaging the step-down power supply or the scan and/or EHT output stages. A well-designed circuit must provide protection in the case of such failures.
Protection of the scan and/or EHT output stage in multiscan displays is presently realized through discrete current limiting circuitry which is usually part of the power supply system. This circuitry, which often monitors current flowing through a sense resistor/current transformer, will interrupt the current if the set trigger level has been crossed. Thus, existing protection systems rely on distinct and separate circuitry, adding to the complexity and cost of the system.